Three different groups of subjects characterized by Combined Dementia Ratings (CDR) will be enrolled in a longitudinal study on the clinical and biomedical aspects of aging and Alzheimer disease. One cohort of elderly subjects, including 57 non-demented control subjects (CDR 0), 13 patients with questionable Alzheimer disease (CDR 0.5), and 26 subjects with mild but definite Alzheimer disease (CDR 1), has already been identified and followed for 2 to 3 years. An additional 25 subjects will be recruited in each of these 3 categories and enrolled in the longitudinal study. In a third project, 25 subjects in each of 3 categories of SDAT severity (CDR 1, 2 and 3) will be tested once in a cross-sectional study for comparison with the longitudinal data. Patients will be recruited from the community; the investigators are not providing diagnostic or continuing care. All subjects are screened according to a standardized protocol (the Initial Subject Protocol) that includes neurological and psychiatric examinations. Subjects fulfilling diagnostic criteria undergo subsequent blood tests, EEG, CT, and PET brain scans, psychological examinations and family interviews according to the research criteria. Autopsies will be sought in all cases and firm diagnoses will be established according to neuropathological criteria.